Field of the Invention
The invention relates to an elongated superconductor structure for carrying an electric current in a predetermined direction. The structure is intended to have the following parts: a biaxially textured mount composed of metallic material, an intermediate layer system which is deposited on the mount and has at least two intermediate layers composed of different oxide materials, and a superconducting layer which is deposited on the intermediate layer system and is composed of a high-Tc superconductor material of the (RE)M2Cu3Ox type. The RE component contains at least one rare earth metal (including yttrium) and the M component contains at least one alkaline-earth metal. The invention also relates to a method for producing such a superconductor structure. A corresponding superconductor structure and a method for producing it can be found in Applied Superconductivity, 1996, Vol. 4, Nos. 10-11, pages 403 to 427.
Superconducting metal-oxide compounds having high critical temperatures Tc of above 77 K are known. They are therefore also referred to as high-Tc superconductor materials or HTS materials. Their particular advantage is that they allow a liquid nitrogen (LN2) cooling technique. Metal-oxide compounds such as these include, in particular, cuprates based on specific material systems such as that of the (RE)-Mxe2x80x94Cuxe2x80x94O type with the RE components containing at least one rare earth metal (including Y), and the M component containing at least one alkaline-earth metal. The main representative of this type is the material YBa2Cu3Ox (referred to as xe2x80x9cYBCOxe2x80x9d).
Attempts are being made to deposit these known HTS materials on different mounts (substrates) for different application purposes. The aim is, in general, to produce a textured superconductor material with as high a phase purity as possible, in order to achieve a high current carrying capacity. The term texturing in this case means the alignment of the crystallite of a polycrystalline structure. In particular, elongated metallic mounts are intended for conductor applications. See, for example, U.S. Pat. No. 4,921,833 (corresponding European EP 0 292 959 A2) to Takano.
In the case of a corresponding superconductor structure for conductor applications, the HTS material is generally not deposited directly on a metallic mount strip used as a substrate; instead, this mount strip is first of all covered by at least one thin intermediate layer, which is also referred to as a buffer layer. This intermediate layer has a thickness in the order of magnitude of about 1 xcexcm and is intended to prevent metal atoms from diffusing out of the mount material into the HTS material, in order to prevent the superconducting characteristics from being adversely affected as a result of this. At the same time, such an intermediate layer, which is used as a diffusion barrier, allows the surface to be smoothed and the adhesion of the HTS material to be improved. Appropriate intermediate layers are composed in particular of oxides of metals such as zirconium, cerium, yttrium, aluminum, strontium or magnesium, or of alloys with these metals, and are thus in general dielectrics.
In addition to the characteristic as a diffusion barrier, this at least one intermediate layer is, furthermore, intended to satisfy the requirements of allowing textured growth of the HTS material to be applied to it. In consequence, the intermediate layer must itself have a corresponding texture. The transfer of the crystallographic orientation during the growth of a layer on a substrate of a chemically different type is known by the term heteroepitaxy. In that case, the unit cells in the intermediate layer must have dimensions which are matched as well as possible to the lattice constants of the HTS material. Furthermore, it should have a thermal coefficient of expansion which at least approximately matches that of the HTS material in order in this way to avoid undesirable mechanical stresses during the temperature cycles, which are unavoidable for applications relating to superconducting technology and for layer preparation, and possible damage such as exfoliation resulting from this.
The choice of the xe2x80x9cmount-intermediate layerxe2x80x9d system is subject to similar requirements. In that case as well, good adhesion characteristics are desirable, and, at the same time, the desired heteroepitaxy between the intermediate layer and the HTS layer which is growing on it must not be adversely affected.
For the reasons mentioned above, the literature reference cited initially provides for a metal strip whose surface is biaxially textured by means of a rolling process and which is composed of copper or nickel to be used as the mount strip, on which an intermediate layer in the form of a CeO2 layer (as the first buffer layer) and a thicker layer composed of ZrO2 stabilized with Y (Zr(Y)O2 as the second buffer layer) are deposited. This combination was chosen since CeO2 can be deposited heteroepitaxially in a reducing atmosphere on textured nickel in order to avoid oxidation of the metal surface. However, the oxygen deficit which results in this case in the CeO2 and the enlargement of the lattice constants associated with this mean that there is a tendency for the layer to form cracks. CeO2 layers with a maximum thickness of only 100 nm are therefore used, in order to restrict the surface density at corresponding cracks. This is because thicker CeO2 layers tend to form cracks more easily than thin layers. This makes it necessary to use a second layer composed of Zr(Y)O2 in order to allow any cracks or other mechanical damage to be covered. This second layer must be made sufficiently thick, for example up to 1 xcexcm, that it represents the actual diffusion barrier. A corresponding intermediate is layer system is described, inter alia, in U.S. Pat. Nos. 5,739,086 and in 5,741,377.
Such a conductor structure is thus subject to the requirement for deposition of crack-free intermediate layers, which are resistant to diffusion, on metal strips. Furthermore, a biaxially textured metal strip must be assumed in order to achieve the heteroepitaxy which is important for high critical current densities Jc, in order in this way to allow the texture of the strip to be transferred to the superconducting material. The aim of this is to allow the production of conductors in the form of strips and having a long length that are coated with superconducting YBCO material or a corresponding HTS material.
High critical current densities Jc in the YBCO layer of at least 1xc3x97105 A/cm2 and with the YBCO material having a layer thickness of, for example, 0.8 xcexcm are required for the applications which have been conceived of so far for loss-free transmission of high currents, for example in the form of wound strips in solenoid coils or transformers, which are then subject to high magnetic fields. This could be achieved only by aligning the crystallites of the YBCO layer in the same way in the crystalline a-b plane since then, as is known, small angle grain boundaries result in a high current carrying capacity. This is also referred to as a biaxial texture. This leads to the necessity for the biaxial structure which already exists in the mount to be transferred through the intermediate layers to the YBCO. What is referred to as the full width at half maximum (FWHM) in an x-ray xe2x80x9cPhi scanxe2x80x9d is used as a measure of the quality of the biaxial texture; this should not significantly exceed 10xc2x0 for the intermediate layer system. Corresponding experiments are described in the Journal of Materials Research, Volume 7, No. 7, July 1992, pages 1641-51, in particular pages 1644-1645.
It is accordingly an object of the invention to provide a high-temperature superconductor structure and a corresponding production method, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which specifies an intermediate layer system that satisfies the requirements mentioned so that even relatively long pieces of mount, in particular of more than 1 m and preferably of more than 100 m, can be coated with a constant quality. The specific technological object is to prevent diffusion of metal atoms from the mount material into the YBCO layer in the same way as diffusion of oxygen, which is required during the deposition and formation of the YBCO, to the metal surface. This is because such oxygen diffusion would lead to metal oxidation at the normal deposition and heat-treatment temperatures for the YBCO material in the range from about 600 to 800xc2x0 C., and would thus decrease the adhesion strength of the intermediate layer. For this reason, intermediate layer thicknesses of between about 0.5 and 2 xcexcm are generally required, with these thicknesses being dependent on the chosen intermediate layer material.
With the foregoing and other objects in view there is provided, in accordance with the invention, an elongated superconductor structure for carrying an electric current in a predetermined direction, comprising:
a biaxially textured mount composed of metallic material;
an intermediate layer system including a first, relatively thicker, intermediate layer of yttrium oxide deposited on the mount and a second, relatively thinner, intermediate layer of cerium oxide above the first intermediate layer; and
a superconductor layer deposited on the second intermediate layer and composed of a high-Tc superconductor material of the (RE)M2Cu3Ox type, with an RE component containing at least one rare earth metal, and an M component containing at least one alkaline-earth metal.
In other words, the above objects are satisfied, in the context of the above-mentioned superconductor structures, with an intermediate layer system having an intermediate layer which faces the mount and is composed of yttrium oxide and a relatively thinner intermediate layer which faces the superconductor material and is composed of a cerium oxide. The main representatives of these oxides are Y2O3 and CeO2. Minor impurities or additives, in particular of other metal oxides, up to a proportion of 5% by weight should, however, in each case also be included.
The advantages associated with this configuration of the intermediate layer system are, in particular, as follows:
The (first) intermediate layer, which faces the mount, can be grown heteroepitaxially on the metallic mount; this can be ensured by the prevention of oxide formation on the metal surface. To this end, the metal oxide in the first intermediate layer must be deposited in a reducing or low-oxygen atmosphere, and must have a considerably higher bonding energy than the metal oxide in the mount material. These requirements can be satisfied by the chosen Y2O3.
The (second) intermediate layer, which faces the superconductor material, has a lattice constant which is very well matched to the chosen superconductor material. The corresponding error (referred to as the lattice mismatch) is, specifically, less than 1.6% when using CeO2, while the equivalent error for Y-stabilized ZrO2 is 5.5%.
The overall intermediate layer system prevents chemical reactions between the metal of the mount material and the superconductor material during the necessary processes for coating the mount. This is because it has a low diffusion rate for the elements involved. Furthermore, it can be configured such that it is reproducible even over great lengths, with no macroscopic defects such as cracks occurring. The chosen combination of intermediate layer materials is, in consequence, particularly suitable for producing an elongated conductor structure with a great length using the materials for its mount and its superconductor.
Advantageous refinements of the superconductor structure according to the invention can be found in the dependent claims relating.
In accordance with an advantageous feature of the invention, a mount composed of nickel or a nickel alloy is chosen. Firstly, this is because mounts composed of materials such as these have a thermal coefficient of expansion which is in the same order of magnitude as that of Y2O3. Secondly, the required biaxial texture can be produced in a manner known per se (see the literature reference cited initially) and relatively easily on the mount surface to be coated.
In a corresponding way, the mount is preferably provided with a rolled texture on its surface facing the intermediate layer system.
The superconductor material is AD2Cu3Ox preferably of the YBa2Cu3Ox (YBCO) type.
It is just as possible to use a material in which, based on YBCO, the Y component and/or the Ba component are/is at least partially replaced by an element from the respectively corresponding group. In accordance with an added feature of the invention, the A component represents yttrium and other elements of its group, and the D component represents barium and other elements of its group. This means, at least one of the yttrium and barium components may be at least partially replaced by an element from their respective group.
The CeO2 intermediate layer advantageously has a thickness of less 200 nm, preferably of 100 nm, and of at least 10 nm. This layer thickness allows good heteroepitaxy to be ensured. Furthermore, it prevents the formation of cracks.
In accordance with another feature of the invention, it is found to be advantageous for the Y2O3 intermediate layer to have a thickness of between 200 nm and 2 xcexcm. Such a relatively thick intermediate layer ensures the denseness of the entire intermediate layer system, even over great superconductor structure lengths.
It is particularly advantageous for at least largely heteroepitaxial growth of the intermediate layers and of the superconducting layer to be ensured for production of the superconductor structure. The full width half maximum (FWHM) in the X-ray phi scan should in this case be less than 20xc2x0 and preferably less than 15xc2x0. This is because it allows a high critical current density Jc to be achieved in the superconducting layer.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an elongated superconductor structure with a high-Tc superconductor material and a metallic mount, and a method for producing this structure, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.